1. Technical Field
The present invention generally relates to the control and containment of gases, and more particularly to containing chemical reactions involving gases.
2. Description of Related Art
A variety of industrial processes create gas streams that must be scrubbed of contaminants before being released to the outside world. The manufacture of electronics, solar cells, display devices, communications devices, metals, ceramics, and polymers, as well as the processing of chemicals, drugs, and other materials, often requires the use of exhaust gas scrubbers. Scrubbers typically receive a substantially gaseous exhaust stream (sometimes containing fine particles) and remove contaminants from the gas stream before the stream is released to the environment.
Exhaust streams from electronic fabrication processes may include a variety of contaminants, including perfluorocarbons (PFCs) and etch gases such as SF6, NF3, CF4, C2F6, C4F8, COF2, and C4F6. Exhaust streams may include toxic hydrides such as AsH3, PH3, P2H4, or B2H6. Exhaust streams may also contain pyrophoric gases such as SiH4, Si2H6, GeH4, and gases such as WF6, SiF4, HCl, BCl3, Cl2, TiCl4, F2, HF, and various chlorosilanes.
Other industrial processes may also create toxic or polluting exhaust streams particular to a material or manufacturing process. Volatile organic compounds (VOCs) may be present in various petroleum refining processes, chemical reaction processes, or other organic synthesis reactors. Room or chamber ventilation (e.g., of a spray painting facility or an environment containing microbes or viruses) may also require exhaust gas scrubbing or other abatement.
Many contaminants require specific scrubbing procedures. Contaminants such as HCl, Cl2, and BCl3 are often soluble in water, and may often be removed using so-called wet scrubbers. Contaminants such as SiCl4, SiH2Cl2, NH4F, WF6, WCl4, and TiCl4 (herein “water-reactive” contaminants) may or may not dissolve in water, depending upon various conditions. These contaminants may also react with water to form solid reaction products, which may clog various flow paths.
Another category of contaminants includes “water-insoluble” contaminants such as SiH4, PFCs such as CF4 and C2F6, SF6, and NF3. Among other deleterious characteristics, many of these contaminants are characterized by a “global warming potential,” which may be hundreds or thousands of times stronger than that of CO2 and reflecting a much stronger behavior as a greenhouse gas in the Earth's atmosphere.
Some contaminants are often abated by combusting the contaminant to form water-soluble reaction products that are then removed by wet scrubbing. Sometimes, such combustion requires high temperatures. For example, NF3 may be combusted at temperatures above 900 degrees Celsius; CF4 may be combusted at temperatures over 1200 degrees Celsius. Other contaminants such as SiH4 may sometimes be reacted simply by exposing the contaminant to an oxygen source.
Water-insoluble contaminants may form reaction products (e.g., HF) that may be removed by wet scrubbing the reacted gas stream. Other water-insoluble contaminants (e.g., SiH4) may form reaction products that include solid species (e.g., SiO2).
Generally, solid species in a waste stream may be present as fine particles in a liquid phase (e.g., water associated with a scrubber), in the gas phase, deposited on a solid surface, or in other ways. These solid species may also nucleate directly on various surfaces. While the formation of solid reaction products may enable certain removal methods (e.g., filtration), these species may also deposit on and clog various lines, inlets, passages, surfaces, and other aspects of the system, reducing the system's efficiency or stopping its operation.
Some gas streams may include a variety of contaminants, including water-soluble, water-reactive, and water-insoluble contaminants. Scrubbing such a mixed gas stream may be particularly challenging. Many processes also create one type of contaminant during one step and another type of contaminant in another step. For example, the exhaust gas stream associated with a deposition tool may include SiCl4 during a deposition step, requiring abatement of SiCl4. The tool may be cleaned with a PFC during a cleaning step, and thus require abatement of the PFC. A preferred abatement system would abate all gas streams exiting a tool, and so an exemplary abatement system might be required to abate both SiCl4 and the PFC.
For gas streams including a variety of contaminants, effective scrubbing may require multiple systems, such as a wet scrubber to remove water-soluble contaminants combined with a combustion chamber to combust water-insoluble contaminants. Often, the presence of one contaminant may impede the ability of a system to remove another contaminant, and for contaminants that form solid reaction products, deposition of these reaction products can be a significant problem, particularly with “downstream” systems. For example, PFCs may be removed by combustion processes in a combustion chamber, but if the incoming gas stream also contains corrosive contaminants (e.g., HCl), the materials in the combustion chamber may be attacked by the corrosive contaminants during combustion of the PFC. Additionally, combustion of the PFC may yield combustion products (e.g., F2, HF or even OF2) that may themselves be toxic, corrosive, and requiring of additional abatement. Combustion of SiH4 may yield solid SiO2 particles, which may deposit on various surfaces and clog the apparatus. Additionally, water-soluble contaminants in the gas stream entering the combustion chamber may deposit, corrode, or otherwise degrade components of the combustion chamber. Often, a preferred system or method for abating a first contaminant in a mixed gas stream creates a problem in a subsequent system for abating a second contaminant. In such situations, a system optimized for the abatement of mixed gas streams may be desired.